1. Field of the Invention
The present invention relates generally to a mobile communications system, and in particular, to an apparatus and method for performing a base station search in an asynchronous mobile communications system.
2. Description of the Related Art
With the rapid development of mobile communication technology, future mobile communication systems will provide a data service and a moving image service as well as the existing voice service. The standardization of such a system is now in progress. The future mobile communications system can be divided into a synchronous mobile communications system led by the United States and an asynchronous mobile communications system led by the European Community. The European asynchronous mobile communications system is commonly referred to as a “Universal Mobile Telecommunications System (UMTS)”.
The asynchronous UMTS system must perform a base station (or cell) search operation to acquire synchronization with a specific base station through given synchronization (sync) channels. The two sync channels used for a base station search in the UMTS system, are included in a downlink physical channel (DPCH). One channel is a primary sync channel (P-SCH) and the other is a secondary sync channel (S-SCH). The SPEC in connection with the UMTS sync channel can be found in ETSI TS 25,211 TS 25,213 Release 99. As illustrated in FIG. 1, the P-SCH has a sequence length of 256 chips and constitutes a first 256-chip period of every slot (1 slot=2560 chips). A mobile station of the UMTS system acquires slot timing synchronization using the P-SCH.
A mobile station of the UMTS system the performs P-SCH search prior to the S-SCH search. After slot timing synchronization by the P-SCH search, frame timing synchronization (Fsync) is acquired and a primary scrambling code group is determined by the S-SCH search. The Fsync and the PSCG determination by the S-SCH search are performed based on the slot timing synchronization by the P-SCH search.
FIG. 2 is a block diagram illustrating a conventional apparatus for performing S-SCH search. The apparatus includes an S-SCH RSSI (Secondary Synchronization Channel Received Signal Strength Indicator) calculator 100, an S-SCH energy matrix update part 102, and an S-SCH searcher 104.
Referring to FIGS. 1 and 2, a conventional S-SCH search operation will be described below. In the UMTS system, one frame has a period of 10 ms and includes 15 slots (SLOT#0-SLOT#14). Each base station is assigned one of 512 primary scrambling codes, and the mobile station must first determine the PSCG in order to find out the unique primary scrambling codes used by the respective base stations. The 512 primary scrambling codes are associated with 64 PSCGs, and each PSCG includes 8 primary scrambling codes (512=64×8). In the 64 PSCGs, a PSCG includes the primary scrambling codes #0-#7, a 2nd PSCG includes the primary scrambling codes #8-#15, . . . , and a 64th PSCG includes the primary scrambling codes #504-#511.
In the S-SCH search operation of the mobile station, the S-SCH RSSI calculator 100 calculates 16 energy values Em,k at an mth slot (m=1,2, . . .) as expressed by Equation (1) below, in order to acquire the Fsync and determine the PSCG. Therefore, the S-SCH RSSI calculator 100 calculates 16 energy values at every slot.Em,k=[Em,k,i ]2+[Em,k,Q]2, k=1, . . . , 16  (1)where             E              m        ,        k        ,        I              =                  ∑                  i          =          0                255            ⁢                           ⁢                                    r            I                    ⁡                      (                          m              ,              i                        )                          ·                              SSC            k                    ⁡                      (            i            )                                ,            and      ⁢                           ⁢              E                  m          ,          k          ,          Q                      =                  ∑                  i          =          0                255            ⁢                           ⁢                                                  r              Q                        ⁡                          (                              m                ,                i                            )                                ·                      SSC            k                          ⁢                              (            i            )                    .                    
In Equation (1), rI(m,i) and rQ(m,i) indicate an ith I-channel signal and an ith Q-channel signal received respectively at the mth slot (where i=0-255), and SSCk(i) indicates an ith chip of a kth SSC (Secondary Sync Code).
The 16 energy values Em,k, calculated by the S-SCH RSSI calculator 100 at every slot, are provided to the S-SCH energy matrix update part 102, which updates a 15×16 matrix S, shown below, using the energy values Em,k. In the matrix S, S(i,j) indicates an element in an ith row and a jth column.
In the initial state: S(i,j)=0, i=1,2, . . . , 15 and j=1,2, . . . ,16
At the mth slot (m=1,2,3, . . . ):
if (m mod 15)==0                i=15;        
else                i=(m mod 15);        
S(i,j)=S(i,J)+E(i,j);
Hereinafter, the matrix S will be defined as an S-SCH energy matrix.
The S-SCH energy matrix, constantly updated by the S-SCH energy matrix update part 102 is provided to the S-SCH searcher 104 when a search start command Start_SEARCH (which is transitioning from ‘0’ to ‘1’) is applied to the S-SCH searcher 104 at predetermined time intervals.
The S-SCH searcher 104 acquires Fsync and determines a primary scrambling code group number PSCG_No by performing the S-SCH search using the S-SCH energy matrix constantly updated by the S-SCH energy matrix update part 102, an SSC table for the S-SCH, illustrated in FIGS. 4A to 4C, and Equation (2) given below. A detailed description will be made below regarding how to acquire the Fsync and determine the PSCG_No.
As a typical method for searching the S-SCH, the S-SCH searcher 104 calculates S-SCH energy for each of the S-SCH patterns associated with the 64 PSCGs (hereinafter, referred to as “64 S-SCH patterns”) in the SSC table illustrated in FIGS. 3A to 3C. Since the Fsync is not acquired during the S-SCH search, the 64 S-SCH patterns illustrated in FIGS. 4A to 4C, shifted by L slots (L=0, . . . , 14), can all become a hypothesis of the Fsync and the PSCG_No. The number of hypotheses searched to acquire the Fsync and determine the PSCG (i.e., the number of hypotheses to be energy-calculated) is 960 (=64×15). The search for a (p,q)th hypothesis (where p=1,2, . . . , 64 and q=1,2, . . . , 15) out of the 960 hypotheses is calculated in (p,q)th S-SCH energy as expressed in Equation (2) below.                                                                         (                                  p                  ,                  q                                )                            th                        ⁢                                                   ⁢            S                    -                      SCH            ⁢                                                   ⁢            energy                          =                              ∑                          l              =              0                        14                    ⁢                                           ⁢                      S                          (                                                l                  +                  1                                ,                                  t                  ⁡                                      (                                          p                      ,                      q                      ,                      l                                        )                                                              )                                                          (        2        )            where t(p,q,l)=SSC of group p at slot ((q−1+l) mod 15) (as illustrated in the SSC table in FIGS. 3A to 3C).
The S-SCH searcher 104 can acquire Fsync and determine a PSCG of the base station by searching the hypothesis having the maximum energy out of the 960 hypotheses, using Equation (2).
However, the conventional apparatus has the following disadvantages:
(1) Searching for all the hypotheses using equation (2) with a same S-SCH observation time requires a long S-SCH search time.
(2) Searching for the hypotheses after observing the S-SCH energy for a predetermined time period (e.g. a 1 or 2-frame period) is inefficient because the channel conditions may very with the passage of time. For example, when the Signal-to-Noise Ratio (SNR) is very low, a very long time period is required for S-SCH energy observation to guarantee high detection probability and low false alarm probability. Thus, in this case, using a predetermined observation may result in a decrease of the detection probability and an increase in false alarm probability. On the contrary, when the SNR is very high, S-SCH energy observation over a short time period can result in good S-SCH search performance. This means that using a predetermined observation time may result in an unnecessary increase in the search time.